This invention relates generally to signal receivers and particularly to signal receivers employing constant false alarm rate (CFAR) circuitry.
As is known in the art, receivers operated in an environment in which the level of extended background signals vary greatly may result in saturation of the receiver's detection apparatus. To avoid the deleterious effect of saturation, such receivers may be adapted to operate as a "Constant False Alarm Rate" (CFAR). In a "CFAR" receiver circuitry is provided to automatically adjust sensitivity as the level of the extended background signals vary, thereby allowing detection of larger "point" target signals regardless of the level of such background signals. The desired automatic adjustment may be accomplished by one of two techniques generally referred to as "amplitude discrimination" and "phase discrimination." As is known, satisfactory application of amplitude discrimination technique requires complex electronic control and filtering circuitry in order to achieve proper response time in the presence of widely varying levels of extended background signals. Such complex electronic circuitry, however, increases the cost of such type of CFAR circuitry. On the other hand, any phase discrimination technique used prior to the present invention is not so dependent on complex circuitry for proper operation. Such CFAR use a phase coded transmitted signal so that received echo signals, after being passed through a limiter to normalize their power prior to their being processed by the detection apparatus, are correlated with the phase coded transmitted signal. The degree of correlation then may be taken as an indication of the nature of the source of each echo signal, i.e. whether a particular echo signal is from a "point" target if an "extended" background object. Implementation of the "phase discrimination" technique requires that a portion of the CFAR circuitry is divided between the transmitter and the receiver and also that a coherent transmitter-receiver system be employed.